Rickettsia rickettsii is the causative agent of Rocky Mountain spotted fever (RMSF), one of the most severe infectious diseases in the United States. To better understand the molecular mechanisms by which rickettsiae undergo transmission to vertebrate hosts, we will characterize rickettsiae inside the tick vector. Our long term goal is to exploit the differences between rickettsiae of varying pathogenicity and elucidate the mechanisms of rickettsial pathogenesis in ticks in order to develop novel approaches to control tick-borne rickettsial diseases. Towards this objective, we propose to use comparative proteomics to assess the molecular interactions between ticks and tick-borne rickettsiae. Aims of these studies will characterize the rate at which spotted fever group Rickettsia spread through individual ticks and throughout individual tick tissues, in addition to identifing Rickettsia-derived molecules that are involved in transmission events. Experiments under these aims will identify: 1) differences in species ranging from endosymbionts to pathogens, 2) differences in rickettsial protein expression profiles in various tissues, and 3) conserved and unique antigens recognized by the vertebrate host. Studies will assess proteomes of Rickettsia rickettsii in the salivary glands and ovaries of Dermacentor variabilis ticks by two-dimensional gel electrophoresis. Additionally, antigenic proteins will be identified by immunoblot analysis; candidate proteins will be identified by mass spectrometry. Comparative analysis will include proteomes of rickettsiae not associated with human disease (R. montanensis and R. bellii). By identifying molecules which define the virulence of rickettsiae in arthropods, novel aspects of rickettsial biology will be elucidated and subsequently exploited for control of the pathogen. Rickettsioses are diseases caused by members of the typhus group and spotted fever group of Rickettsia. Pathogenic Rickettsia are typically transmitted to humans by arthropods (insects and ticks). To better understand the mechanisms of rickettsial virulence, this project seeks to examine the interplay between spotted fever group Rickettsia and the vector tick. [unreadable] [unreadable] [unreadable]